I bought a HU800HW push mower and when I disengage the wheels so it can go backwards, it keeps locking up. But it will go forward just fine just having issues going backwards... what could be the solution?

Hi,
Your owners manual is at this link:

HU800HW_532438221.pdf (http://www.husqvarna.com/ddoc/HUSO/HUSO2010_NAen/HUSO2010_NAen_HU800HW_532438221.pdf)
And that second nine digit number being their internal control number.
Page ten tells of the wheels not turning freely because of trash buildup. Page 13 shows the transaxle and some of what it takes to get to it. Most everything sensitive is behind the drive wheels.
Peace,
Clarke